Acrylic fibers with improved brightness and process for producing the same

ABSTRACT

An acrylic fiber with a triangular cross-section wherein the curved peripheral edge thereof is defined in accordance with formulas discussed herein. Also provided is a process for preparing these fibers, which exhibit improved brightness, by extruding a spinning solution through Y-shaped orifices into a coagulating bath, the inorganic salt concentration in the spinning solution and coagulating bath being within defined ranges, maintaining the delivery ratio within the range 0.5-3.0 and subjecting the filament to a series of stretching steps which include cold-stretching, primary hot-stretching and secondary stretching such that 16X1 + 2.5X2 - 3X3 &lt; OR = 31 where X1 is the cold-stretching ratio, X2 is the primary hotstretching ratio and X3 is the secondary stretching ratio. This invention relates to acrylic fibers with improved brightness and a process for producing the same. There have been already made many attempts to improve the brightness of synthetic fibers by forming their cross-section in a noncircular shape (known as fibers of differently shaped crosssections). For example, in the U.S. Pat. No. 2,939,201 or No. 2,939,202, it is mentioned that a textile filament of a crosssection having substantially symmetrical three lobes has an excellent brightness. However, the technique disclosed therein is directed solely to a melt-spinning system which can maintain a comparatively high degree of correlativity between the crosssectional shape of the spinning orifice and that of the spun fiber, but has not been applicable in fact to a wet-spinning system wherein the cross-sectional shape of the fiber is determined by the correlation between the desolvating velocity when the spinning solution is extruded into the coagulating bath and the diluting velocity of the solvent and by the mechanism of the formation of a skin-core structure. As a result of researches to impart a silky brightness to acrylic fibers obtained by a wet-spinning method and to improving their esthetic value, we have found that the brightness of acrylic fibers represented quantitatively as the later mentioned 60-degree mirror-surface luster degree and contrast index is not to be determined on such individual factors as the solvent concentration and streching ratio in the coagulating bath, but can be attained only by adopting limited spinning conditions in a series of fiber producing steps from the extrusion of the spinning solution into the coagulating bath to the steam heat stretching. A principal object of the present invention is to provide a special form peculiarity of acrylic fibers remarkably improved in the brightness. A second object of the present invention is to provide a new process to obtain acrylic fibers having a remarkably improved brightness represented quantitatively as a correlation with the 60-degree mirror-surface luster degree and contrast index.

United States Patent Shimoda et al.

[ 1 June 27, 1972 154] ACRYLIC FIBERS WITH IMPROVED BRIGI-ITNESS AND PROCESS'FOR PRODUCING THE SAME [72] Inventors: Keitaro Shimoda; Nobuhiro Tsutsui; I-Iideto Seklguchi; Masao Sone, all of 211 Appl. No.: 7,858

[30] Foreign Application Priority Data Feb. 3, 1969 Japan ..44/8366 [52] US. Cl ..161/177, 264/177, 264/182, 1 264/210 F, 264/290 R [51 Int. Cl. .B29f 3/00, D02g 3/22 [58] Field of Search ..161/177; 264/177, 182, 210 F, 264/288, 290

[56] References Cited UNITED STATES PATENTS 3,384,694 5/1968 Nakayama et a1. 3,523,150 8/1970 Vigneault ..264/210 F v 2,948,581 8/1960 Cummings, Jr ..264/182 3,194,002 7/1965 Raynolds et a1. ..161/177 2,777,751 1/1957 Cresswell et a1. ..264/182 3,491,179 1/1970 Chinai ctal ..264/182 FOREIGN PATENTS OR APPLICATIONS 9,314 1963 Japan ..264/182 Primary Examiner-Robert F. Burnett Assistant Examiner-l3orraine T. Kendell Atmmey-Wenderoth, Lind & Ponack 57 ABSTRACT An acrylic fiber with a triangular cross-section wherein the curved peripheral edge thereof is defined in accordance with formulas discussed herein. Also provided is a process for preparing these fibers, which exhibit improved brightness, by extruding a spinning solution through Y-shaped orifices into a coagulating bath, the inorganic salt concentration in the spinning solution and coagulating bath being within defined ranges, maintaining the delivery ratio within the range 0.5-3.0 and subjecting the filament to a series of stretching steps which include cold-stretching, primary hot-stretching and secondary stretching such that 16 X +2.5 X 3 X 31 where X is the cold-stretching ratio, X is the primary hotstretching ratio and X is the secondary stretching ratio.

3 Claims, 4 Drawing Figures ACRYLIC FIBERS WITH IMPROVED BRIGH'INESS AND PROCESS FOR PRODUCING THE SAME This invention relates to acrylic fibers with improved brightness and a process for producing the same.

There have been already made many attempts to improve the brightness of synthetic fibers by fonning their cross-section in a noncircular shape (known as fibers of differently shaped cross-sections). For example, in the US. Pat. No. 2,939,201 or No. 2,939,202, it is mentioned that a textile filament of a cross-section having substantially symmetrical three lobes has an excellent brightness. However, the technique disclosed therein is directed solely to a melt-spinning system which can maintain a comparatively high degree of correlativity between the cross-sectional shape of the spinning orifice and that of the spun fiber, but has not been applicable in fact to a wet-spinning system wherein the cross-sectional shape of the fiberis determined by the correlation between the desolvating velocity when the spinning solution is extruded into the coagulating bath and the diluting velocity of the solvent and by the mechanism of the formation of a skin-core structure.

As a result of researches to impart a silky brightness to acrylic fibers obtained by a wet-spinning method and to improving their esthetic value, we have found that the brightness of acrylic fibers represented quantitatively as the later mentioned 60 -degree mirror-surface luster degree and contrast index is not to be determined on such individual factors as the solvent concentration and streching ratio in the coagulating bath, but'can be attained only by adopting limited spinning conditions in a series of fiber producing steps from the extrusion of the spinning solution into the coagulating bath to the steam heat stretching.

A principal object of the present invention is to provide a special form peculiarity of acrylic fibers remarkably improved in the brightness.

A second object of the present invention is to provide a new process to obtain acrylic fibers having a remarkably improved brightness represented quantitatively as a correlation with the 60-degree mirror-surface luster degree and contrast index.

Therefore, the present invention includes two subject matters as evident from the following detailed explanation. One of them is a form peculiarity defining the cross-sectional shape of acrylic fibers made by a wet-spinning method and the other is a new fiber producing process for imparting such form peculiarity to acrylic fibers by a wet-spinning method.

The first object of the present invention can be attained by forming a predetermined curved peripheral edge defined by the following relative formulas in a triangular cross-section of an acrylic fiber produced by a wet-spinning method as exemplified in FIG. 4. Thus, there is provided an acrylic synthetic fiber having a predetermined curved peripheral edge shape defined by the relative formula wherein I is a length (in microns) of one side represented a; a straight line connecting two apexes of a triangular cross-section of an acrylic fiber, a is a minimum curve height (in microns) of the peripheral edge measured in the direction vertical to the straight line connecting the two apexes of the above mentioned triangle, p is a minimum curve pitch length (in microns) of the peripheral edge measured in the direction along the straight line connecting the two apexes of the above mentioned triangle and d is a monofilament fineness (in deniers) of the acrylic fiber having the triangular cross-section.

The second object of the present invention is attained by adopting the later described particular fiber producing conditions in the wet-spinning of acrylic fibers by using a spinnerette having Y-shaped or approximately Y-shaped spinning orifices.

The above described and other objects of the present invention will be made clearer by the following description.

First of all, in order to facilitate understanding of the brightness of acrylic fibers in the present invention, the results of precise experiments made by us in respect of the brightness of acrylic fibers having a triangular cross-section will be explained in the following. The acrylic fibers referred to in the present invention mean single-component fibers consisting of an acrylonitrile homopolymer or of a copolymer of at least percent by weight acrylonitrile and one or more monomer compounds copolymerizable with acrylonitrile or composite fibers made by bonding two components (polymers or copolymers) different in the thermoshrinking behavior in a core-sheath form or side-by-side form. The fibers may be in the form of staples or monofilaments or multifilarnents spun in the form of continuous filaments.

We have noted the fact that not only the macroscopic shape of the cross-section of the fiber but also the microscopic structure of the surface or interior of such fiber should be considered as a factor controlling the brightness of fibers. The brightness of fibers has been so far considered only as based on the macroscopic form peculiarity of a fiber filament of a comparatively smooth surface condition obtained by a meltspinning system, as discussed for example in US. Pat. No. 2,939,201 or No. 2,939,202. However, there are some difficulties, in fact, in simply handling the brightness of fibers made by any spinning system in light of the macroscopic shape of the fibers. For example, the means of imparting a brightness to fibers to be made by a wet-spinning system wherein the cross-sectional shape in which the peripheral edge of the fiber is curved is determined by the correlation between the desolvating velocity after the spinning solution is extruded into the coagulating bath and the diluting velocity of the solvent and the formation of a skin-core structure, can not be considered to be equivalent with the technique of imparting a brightness to fibers to be made by a melt-spinning system or dry-spinning system wherein a smooth peripheral shape is made and a considerable continuity is recognized between the cross-sectional shape of the spinning orifice and that of the fiber.

As a result of repeating experiments on the basis of such viewpoint, we have confirmed that the factors of imparting a silky brightness to acrylic fibers by a wet-spinning system wherein a peculiar curved peripheral edge surface is formed are found in the macroscopic form peculiarity of the crosssection of an acrylic fiber forming a triangle as a whole and the fine curved form on the peripheral edge surface of said triangular cross-section as defined by the relative formulas (1) and (2) and that such brightness can be quantitatively determined as a correlation between the contrast index of the fiber defined by the later described log (h /I and the 60- degree mirror-surface luster degree defined by .lIS 2-8741 as a mirror-surface luster of an angle of incidence of 60 degrees.

The following description will 'be made partly by referring to the accompanying drawings wherein FIG. 1 is a system view of a contrast index measuring apparatus, FIG. 2 is a plan view of the apparatus showing the formation of the essential part in FIG. 1, FIG. 3 is a graph for explaining the shapes of peaks on a recording paper representing ahigh-lightness of a standard knitted texture, and FIG. 4 is a magnified cross-sectional view of an acrylic synthetic fiber of the present invention.

I-Iere, first, the 60-degree mirror-surface luster degree will be explained. The 60-degree mirror-surface luster degree in the present invention was measured according to the method defined in JIS Z-874l by using a GM-5 luster meter (manufactured by Murakami Color Technical Laboratory). A rectangular test piece of 6 cm X 4.5 cm of a cardboard on which fiber bundles having had the crimps tensioned and stretched under heating were parallelly arranged andpasted was prepared and was measured in a position where the angle of incidence was parallel with the axial direction of the fiber.

Now, the contrast index meaning the brightness based on the macroscopic form peculiarity of a fiber in the present invention will be explained. The contrast index is determined by measuring a zarnple S with a scanning type microscopic luster meter consisting of an electric source stabilizer ST, voltage stabilizer EST, light source L, magnifying lens optical system EL, movable photoelectric multiplying tube R, amplifier A and recorder system REC shown in FIG. 1. Referring more particularly to FIG; 2, the light source is formed of an incadescent bulb of 12-V 30W positioned so that the angle of light incidence to the warp of the knitted texture angle of light incidence to the warp of the knitted texture sample 10 may be 45 degrees and a slide projector 11 having a lens of a focal distance of 54 mm. The light source is operated with a constant voltage controlled by the electric source stabilizer ST and voltage stabilizer EST and is positioned so that, when light is focused on the sample, the distance between the lens center of the slide projector l I and the reflecting point of the light on the surface of the sample S to be measured will be 54 mm. The voltage is made constantby the electric source stabilizer ST and voltage stabilizer EST, the photoelectric multiplying the tube 3 moving in a direction intersecting at right angles with the paper surface in FIG. 2 is positioned in the same plane as of the light source and the sample knitted texture 10 is positioned so that its warp direction is parallel with said horisontal surface. If the direction of a normal erected on the surface of v the sample knitted texture 10 is at angle of degree, the angle of reflection to the yarns forming the sample knitted texture 10 will be able to be defined as 0 degree. On a disc-shaped sample stand 9 provided rotatably in a plane parallel with the above mentioned horizontal plane, the sample knitted texture 10 is so positioned that its surface intersects at right angles with the above mentioned horizontal plane and coincides with the intersection of the respective extensions of the center lines of the light source and photoelectric multiplying tube 3. The size of the knitted texture sample 10 is 450 mm. long in either of the warp and weft directions and the contrast is measured while moving the photoelectric multi-lying tube 3 in a direction parallel with the weft direction along the sample knitted texture surface 0.018 mm. long as measured at right angles with the weft direction and 3.6 mm. long as measured along the weft direction. For this purpose, a magnifying lens opticalsystem EL comprising an objective 8 of a focal distance of 10 mm. and magnification of 11.1 times, lens barrel 7 and camera obscura 6 is provided between the photoelectric multiplying tube 3 and the knitted texture sample 10. The photoelectric multiplying tube 3 having a slit 4 which is 0.2 mm. long along the weft direction and 1.0 mm. long along the warp direction to form a fixed yarn area to be measured is provided as connected with a stage 13 having a variable speed gear box 2 fed with a power from a variable speed motor 1 for reciprocating scanning at a fixed velocity. The weft direction of the knitted texture sample 10 and the moving direction of the stage 13 are made to intersect at right angles with each other and the photoelectric multiplying tube 3 is moved by 40 mm. while maintaining a constant velocity of 8 mm. per minute along the yarn image on a frosted glass fixed to the camera obscura 6. The electric output of the photoelectric multiplying tube 3 is fed through an amplifier A whose voltage is made constant in advance by the high voltage controlling device EST and low voltage controlling device ST in the same manner as in the photoelectric multiplying tube 3 or light source L to such recorder having a reverse electromotive force voltage and a full scale width of 0 to 2 mV. by the voltage divider in the recorder system REC as, for example, an LER-l2A recorder of Yokokawa Electric Machinery Manufacturing Company and is recorded on a recording paper moving while maintaining a constant velocity of 60 mm. per

minute in said part.

Thus, as shown in FIG. 3, the fine high lights reflected from the yarn surface of the knitted texture sample will be recorded as peaks H H H H and the dark parts between the high lights will be recorded as valleys L L L L,, between the peaks respectively on the recording paper. In FIG. 3, if the average heights per unit length of the recording paper of the peaks and the valleys between the peaks are i=n 2 Hi i=1 n h n =10O log Contrast index= log By the way, the contrast index of the knitted texture sample is directly determined by the cross-sectional area and surface shape of the monofilament forming the knitted texture but is also influenced by such secondary factors as the fineness, crimped degree, yarn fineness and number of twists. In the present invention, in order to prevent the influences by these secondary factors from contributing to the contest index, a structure knitted texture made by forming filaments or staples of 3 deniers into 52-denier variously twisted twin yarns of upper twists of 410 T/m and lower twists of 610 T/m and making the warp and weft densities respectively 17 yarns/25 mm. with a knitting machine of 12 gauges is used as a standard knitted texture sample. In the case of filaments, a plain structured texture with 30 yams/25 mm. of warp density and 200 yarns/25 mm. of weft density, made of 400 deniers of non-texturized filaments with 2.5 or 3.0 single filament deniers knitted by a knitted machine with 12 gauges is used. The larger this contrast index, the clearer and stronger the microscopic spot light pencil reflected from the surface of the knitted texture sample and the more remarkable the visional brightness.

According to observations made by us, even in case the macroscopic shape of the cross-section of a fiber is formed to be triangular as said conventionally, unless the microscopic curved peripheral shape on the peripheral edge of the fiber having a triangular cross-section is improved, it will be difficult to expect to improve the contrast property showing the improved microscopic curved peripheral edge shape of the fiber defined by the relative formulas l and 2 is formed or not is judged by the numerical value of the 60-degree mirro-surface luster degree defined by JIS 2-8741. It was confirmed that, in case this numerical value showed more than 35, the microscopic curved form defined by the relative formulas l and 2 was formed in the peripheral edge part of the triangular fiber of the present invention. In case the peripheral edge shape of the fiber made by the process of the present invention was provided with such form peculiarity, the brightness shown by the final product made of the obtained fibers made the contrast index 15 to 50 or more preferably 20 to 40 and the fact that a very optically excellent silky brightness was shown was also made clear. What is to be noted here together with the contrast property is the 60-degree mirror-surface luster degree defined by JIS Z-8741 and representing the brightness based on the peculiarity of the microscopic form of the fiber. Only by adopting the specific spinning conditions clarified to attain the later described second object of the present invention, it is possible to make the 60-degree mirror-surface reflection degree defining the quantitative level of the brightness correspond to the qualitative level of the brightness defined as the above mentioned contrast index.

It is recommended that the value of the 60-degree mirrorsurface luster degree in the present invention is at least 35. It is recognized that, in this case,.the contrast index takes a value of 15 to 50 or more preferably 20 to 40. Only when these two conditions are satisfied simultaneously, it will be possible to impart a silky calm brightness to the acrylic fiber having a triangular cross-section.

The acrylic fiber by a wet-spinning system having such characteristics can be attained by imparting a specific crosssectional shape represented by the following relative formulas to an acrylic synthetic fiber by adopting a series of limited spinning conditions clarified as a means of attaining the second object of the present invention.

That is to say, when the microscopic form peculiarity represented by the relative formulas 2) 8- and IH d S t a vs L and (Ix 3 16 is imparted to an acrylic fiber made by a wet-spinning method and having a triangular cross-section as a whole, the brightness of the fiber will be remarkably improved and the 60-degree mirror-surface luster degree showing the optical reflecting activity of the monofilament and the contrast index showing the brightness sensibly understood in the form of the final product will be both kept in a desirable numerical value range.

Now, the fiber producing conditions for imparting the peculiar cross-sectional shape defined by the relative formulas (l) and (2) to an acrylic fiber, that is, the concrete means of attaining the second object of the present invention shall be described in detail.

The second object of the present invention is attained by spinning a spinning solution into a coagulating bath at a temperature of to C. byadjusting the inorganic salt concentrations in the spinning solution and coagulating solution to be in a range of the relative formulas (wherein y is an inorganic salt concentration (in percent) in the spinning solution and x is an inorganic salt concentration (in percent) in the coagulating bath) and maintaining the delivery ratio defined as the average flow velocity of the spinning solution through the spinning orifice/the pull-out velocity of the coagulated fiber filament in a range of 0.5 to 3.0 in wet-spinning an acrylic fiber by using a spinnerette having Y-shaped or approximately Y-shaped spinning orifices and then imparting to the fiber filament a stretching represente by the following relative formula l6X,+2.5X -3X 31 (7) (wherein X 1 is a cold-stretching ratio at the room temperature, X is a primary hot-stretching ratio in hot water and X isa secondary stretching ratio by steam heat).

That is to say, by adopting the above mentioned limited fiber producing conditions, there can be produced an acrylic fiber having a triangular cross-section in which the peripheral edge part is finely curved and the contrast index and 60- degree mirror-surface reflection degree are both kept in a proper range.

According to the finding by the present inventors, when the contrast index defined by 100 log(h,,/I,,) based on the macroscopic form peculiarity of the cross-section of a fiber is taken in a range of 15 to 50 or more preferably to 40 and the value of the 60-degree mirror-surface luster degree based on the microscopic form peculiarity is made at least 35, a silky calm brightness will be imparted to the acrylic fiber. Here, the value of the 60-degree mirror-surface can be represented quantitatively as a function of the stretching multiplication in a series of stretching steps from the cold-stretching to the secondary stretching by steam heat. According to the results of experiments made by the present inventors, a high degree of correlation was recognized between the measured value L of the 60-degree mirror-surface luster degree and the stretching multiplication and it was clarified that the assumed value L,, of the 60-degree luster degree could be represented by the following formula (6) as a function of the individual stretching multiplications X X and X That is to say, in order to keep the value of the above men tioned contrast index in the range of 15 to 50 or more preferably 20 to 40 as recommended in the present invention, it is necessary to maintain a determined curved form in the peripheral edge part of the cross-section of the fiber and to maintain the value of the 60-degree mirror-surface luster degree which is a brightness based on the microscopic form peculiarity to be at least 35.

As a means of attaining this object, it is proposed to adopt a series of stretching steps defined by the following relative formula 16X,+2.5X,3X;, E 31 (7) derived from the above mentioned relative formula (6), Further, it is nothing deviating from the spirit of the present invention to apply desolventing water washing to the fiber filament having left the coagulating bath or having been coldstretched according to a normal process in such fiber producing step, to provide a drying step between the primary stretching in hot water and the secondary stretching by steam heat or to apply a relaxing heat-treatment under a proper temperature condition to the fiber filament after the secondary stretching.

The fact to be noted is that, when a series of stretching conditions defined by the above mentioned relative formulas are adopted, the value L of the 60-degree mirror-surface reflection degree shown by an actual fiber and the value L,, of the theoretical 60-degree mirror-surface reflection degree calculated from the above mentioned relative formula (6) will show a very high correspondence within :6.

It is needless to say that, in the production of acrylic fibers by a wet-spinning method, in case a series of fiber producing conditions from spinning to stretching deviate from the range which can be taken by the above mentioned integrally combined steps, it will not be able to be expected to form a peculiar peripheral edge shape with which the brightness is remarkably improved and naturally to obtain a desirable correspondence of the 60-degree mirror-surface luster degree and the contrast index to each other.

In working the present invention, it is preferably that the temperature of the spinning solution is in a range of 40 to C, If the spinning solution temperature is lower than 40 C., the back pressure on the spinnerette will rise and the continuation of smooth spinning will be obstructed. In case the spinning solution temperature is higher than 80 C., the fiber forming composition will be colored and the commodity value of the obtained fiber will be remarkably reduced.

Further, in working the present invention, in case the coagulating bath conditions deviate from the lower limit defined by the relative formulas (4)'and (5), the coagulating velocity of the spinning solution will be accelerated in excess, a skin-core structure will be formed and it will be obstructed to impart a detennined curved peripheral edge in the first object of the present invention to the acrylic fiber. On the other hand, it is known that, in case the coagulating bath conditions exceed the upper limit of therelative formulas (4) and (5 the coagulating velocity of the fiber filament will reduce remarkably, the fiber filaments will be glued to each other and continuous spinning will be obstructed and, even in case the spinning is continued, the obtained gelly fiber filaments will be water-washed as uncoagulated, will be therefore devitrified and will be reduced in the quality'.-ln the present invention, it is necemary to take a range of 0.5 to 3 as a preferable deliverty ratio of the spinning solution. Such delivery ratio is required particularly for the object of maintaining a stabilized spinning. Usually, in case the delivery ratio exceeds 3, even if a Y- shaped or approximately Y-shaped spinning orifice shape is adopted, the cross-sectional shape of the actually obtained fiber filament will be approximately circular and it will be difficult to form a microscopic form peculiarity of the cross-section of a fiber known as a so-called different shape cross-section fiber.

The fiber filament having passed through the coagulating bath is then subjected to a series of stretching steps defined by the relative formula (7), that is, the cold-stretching at the room temperature, the primary hot-stretching in hot water and further the secondary stretching in a steam heat medium. Among these stretching steps, the cold-stretching and primary hot-stretching give a substantial elongation to the fiber filament without destroying the skin-core structure formed mostly in the coagulating bath step, accelerate the orientation of the fiber forming components, give a required strength of the fiber filament and become factors of forming a proper curved shape defined by the relative formulas (1) and (2) in the peripheral edge part of the fiber filament. On the other hand, the fiber filament subjected to the primary hotstretching is dried according to a normal process, is then led into a steam heat medium and is subjected to the secondary stretching. Such secondary stretching is to further stretch the fiber filament having the curved peripheral edge shape formed by a series of steps from the coagulating bath to the primary stretching in hot water so that a desirable 60-degree mirrorsurface luster degree and contrast index may be imparted to the finally obtained acrylic fiber. It will be understood from the re1ative formula (6) or (7) that, in order to elevate the value of the 60-degree mirror-surface luster degree, it is efiective to set the coal stretching ratio and the primary stretching ratio in hot water to be as small as possible and to set the secondary stretching ratio by steam heat to be as large as possible and particularly to make the cold stretching ratio small.

The fact that the calculated value L,, of the 60-degree mirrot-surface luster degree defined by the cold-stretching ratio X parimary hot-stretching ratio X in hot water and secondary hot stretching ratio X: by steam heat shows a very favorable correspondence within :6 to the actually measured value L of the 60-degree luster degree shown by an acrylic fiber having a peculiary cross-sectional shape based on the present invention and also shows a favorable correspondence to the value of the contrast index of 100 log( h ll which is a brightness based on the macroscopic form peculiarity of the fiber cross-section will be a finding to be noted in respect that the quantitative evaluation and qualitative evaluation of the brightness can be anticipated totally.

As monomer compounds copolymerizable with acrylonitrile forming an acrylonitrilic fiber in the present invention, there can be enumerated such acrylic esters as methyl acrylate and ethyl acrylate, such methacrylic esters as methyl methacrylate and ethyl methacrylate, such carboxylic vinyl esters as vinyl formate and vinyl acetate, styrene, a-methylstyrene, vinyl chloride, vinylidene chloride, methacrylonitrile, such amides as acrylamide, methacrylamide and a-methylene gretalamide and their N-substituted derivatives, such unsaturated monoand dicharboxylic acids as acrylic acid, methacrylic acid, itaconic acid and maleic acid and their salts, such unsaturated sulfonic acids as allylsulfonic acid, methallylsulfonic acid and styrene sulfonic acid and such unsaturated monomers containing basic nitrogen as vinylpyridine and its derivatives, vinyl pyrrolidone, dimethylaminoethyl methacrylate, 2-hydroxy and 3-methacryloxypropyltrimethylammonium chloride. Further, as aqueous solution of inorganic salts to be used as solvents and coagulating solutions inthe present invention, there can be mentioned aqueous solutions of such thiocyanates as sodium thiocyanate, potassium thiocyanate, calcium thiocyanate and ammonium thiocyanate. Further, such salts as zinc chloride, calcium chloride, sodium perchlorate and potassium perchlorate can be also used.

The present invention has disclosed that the brightness of an acrylic fiber by a wet-spinning system can be determined by the correspondence of the qualitative and quantitative levels of the brightness based on the macroscopic and microscopic form peculiarities of the cross-section of the fiber and a concrete means of imparting such peculiar cross-sectional shape and brightness to acrylic fibers and is a very significant finding in which the brightness which has been rather argued only as a characteristic of synthetic fibers by a wet spinning system is systematized so as to be applicable even to the improvement of the quantitative and qualitative evaluations of acrylic synthetic fibers by a melt-spinning system having a peculiar cross-sectional shape. Further, as an additional effect of the present invention, there can be mentioned the improvement of the. touch and bulkiness based on the form peculiarity of the fiber cross-section.

These surprising effects of the present invention can be attained only by integrally combining the above mentioned constituting requirements and give a suggestion very effective to the improvement of the brightness of acrylic fibers by a wet spinning method which has not yet been substantially clarified.

Examples of the present invention are mentioned in the following. However, the present invention is not limited at all by the descriptions in these examples in which, unless otherwise specified, the percentage is based on the weight as a rule.

EXAMPLE 1 Two kinds of spinning solutions of a copolymer concentration of 11.3 percent and NaSCN concentration of 37 percent prepared by respectively dissolving in a concentrated aqueous solution of NaSCN a copolymer made by copolymerizing acrylonitrile, methyl acrylate and sodium methallylsulfonate at a ratio of 91/8.73/0.27 and a copolymer made by copolymerizing acrylonitrile and vinyl acetate at a ratio of 89/11 as acrylic fiber forming components were heated to a temperature of C. and were delivered into a coagulating bath consisting of an aqueous solution of 12 NaSCN at a temperature of 3 C. by using a spinnerette having the below mentioned cross-sectional shape.

As a spinning device, there was used a gold-platinum spinnerette in which were made 50 Y-shaped orifices in each of which three rectangular slots were radially arranged so that the phase difference might be 120 degrees,the length from the center of the crossing zones of the radial slots to the end of the slot in the lengthwise direction was 0.15 mm. and the width of the slot was 0.003 mm. The value of the delivery ratio of said spinning solution in the coagulating bath was made 1.8. The final spinning velocity through the outlet of the steam stretching machine was made 140 m./min. Thus the spun fiber filament was subjected to the coagulation in the coagulating bath, cold-stretching at the room temperature, water-washing, primary hot-stretching in boiling water, drying under the temperature conditions of a dry-bulb temperature of C. and wet-bulb temperature of 70 C., collapsing, secondary stretching by steam heat at C., relaxing heat-treatment in a steam atmosphere at 127 C. and drying treatment under a dry-heating action at 105 C. to make an acrylic fiber having a triangular cross-section.

Here the cold-stretching ratio X primary stretching ratio X and secondary stretching ratio X were varied. The correspondence to the 60-degree mirror-surface luster degree of the obtained acrylic fiber is shown in the following Table 1.

TABLE 1 Measured value Assumed value Total L of fio degrees L 01 GO-degree stretchmirror-surface mirror-surface N0. Fihr-r forming com poncnts X1 X X ing ratio luster degree luster degree 1, Acrylonitrile/muthyl acrylate/sodium mcthallylsullolmto. 1. 5 1. 5 4. 44 10 58 52 2"... d0 1.5 2 3.33 10 53 47 1. 5 3 2. 22 10 46 41 TABLE 1 Conti nued Measured value Assumed value Total L of fi degrees L of 60-rlegree stretch mirror-surface mirror-surface X X ing ratio luster degree luster degree consisting of an aqueous solution of NaSCN a a temperature of -3 C. while maintaining a delivery ratio of 1.5 of the spinning solution. As a spinnerette, there was used a goldplatinum spinnerette having 4 l 80 Y-shaped orifices in each of which three rectangular slots were radially arranged so that the phase difierence might be 120 degrees, the length from the center of the intersection of the radial slots to the end of the slot in the lengthwise direction was 0.16 mm. in each of the TABLE 2 X X2 X Measured Cold Stretch- Steam Total value L 0160- Cross-sectional stretching ratio stretehstretch degree mirror- Con- Delivshape (mm.)

ing in hot ing ing surface luster trast ery No. ratio water ratio ratio degree index ratio d l p a It will be understood from a series of these test results that, in case the spinning solution and coagulating bath conditions defined by the formulas 4 and 5 and the stretching conditions defined by the formula 7 are not saiisfied, the measured value of the 60-degree mirror-surface luster degree will deviate from the desirable range in the present invention and therefore the two sides and 0.20 mm. in the remaining one side and the width of each slot was 0.03 mm. The spinning conditions except the stretching ratios were made the same as in Example 1. The tendency of the 60-degree mirror-surface luster degree with the variation of the stretching multiplication is shown in Table 3.

contrast property of the fiber will also reduce.

TABLE 3 X X X Measured Stretehvalue L of Assumed value Cold ing ratio Steam Total SO-degree L of 60-degree stretchin hotstretchstretchmirror-surface mirror-surface No. ing ratio water ing ratio ing ratio v luster degree luster degree EXAMPLE 3 65 A spinning solution of a copolymer concentration of 1 l per- EXAMPLE 2 cent and NaSCN concentration of 48 was prepared by dissolving in a concentrated aqueous solution of NaSCN a copolymer made by copolymerizing acrylonitrile, methyl acrylate and dimethylaminoethyl methacrylate at a ratio of 88/9/3 as acrylic fiber forming components, was maintained at a temperature of 50 C. and was spun into a coagulating bath of an aqueous solution of 12 percent NaSCN. As spinnerettes, there was used a spinnerette which was made of a polycarbonate and in which were made 3108 spinning orifices having the same opening shape as in Example 1 and a spinnerette of the same material in which were made 15,000 circular openings of a diameter of 0.095 mm. An acrylic fiber of 3 deniers was made according to all the same steps as in Example 1 except that the spinning solution delivery ratio and stretching ratios were varied respectively as shown in Table 4. The physical properties of the obtained fibers are also mentioned in Table perature, primary hot-stretching in hot water and secondary stretching in a steam heat medium, consecutively, such that 16X, 2.5X 3X, 31 wherein X, is the cold-stretching ratio at room temperature, X is the primary hot-stretching ratio in hot water and X is the secondarystretching ratio in the steam heat medium 2. An acrylic fiber prepared by a wet-spinning method TABLE 4 Number Cross-sectional shape of fiber: Triangular Triangular Circular Circular Spinnerettc 3,108 Y- 1,500 orifices of a 1,5000 orifices of a orifices orifices diameter of diameter of 0.095 mm. 0.095 mm.

Delivery ratio 2. 5 2.0 Cold-stretching ratio: X1 1. 5 2 1. 5 2 Stretching ratio in hot water: K2,. 2. 7 7 2. 7 5 Steam-stretching ratio: X3" 5 1 5 1 Fineness (deniers) 3.0 3.0 3.0 3.0 Dry-strength (g./d.) 4. 3 3.4 4. 3 3. 6 Dry-elongation (g./d.) 30 27 34 44 Knot strength (g./d.)- 2.6 2.1 3.3 2.9 Knot elongation (percent) 16 19 24 34 Young's modulus (g./d.) at the normal te1np. 50 30 50 45 Young's modulus (g./d.) in hot water at 95 C 0. 95 0.60 0.95 0.6 Contrast index 22 19 15 60-degree mirror-surface luster degree 53 55 Glare Present Absent Absent Absent Fonn peculiarity:

What we claim is: characterized by a curved peripheral edge in the triangular l. A process for producing acrylic fibers with improved cross-section of the fiber which edge is defined by the relative brightness which comprises extruding a spinning solution o u through essentially Y-shaped spinning orifices into a coagulatm 1 1 ing bath at a temperature of l0 to 15C., the inorganic salt Z and T 8 (1) concentration in the spinning solution and coagulating bath p p being in the range of fl fi and l 16 35 5 549 d wherein I IS the length in microns of one side of the fiber 7 represented as a straight line connecting two apexes of the tri- 2 l53 angular cross-section of the fiber, a is the minimum curve 47- height in microns of the peripheral edge measured in the direction vertical to the straight line connecting the two apexes of the triangular cross-section, p rs the minimum curve pitch length in microns of the peripheral edge measured in the respectively, wherein y is the inorganic salt concentration, in direction along the straight line connecting the two apexes of percent, in the spinning solution and x is the inorganic salt the triangular cross-section and d is the monofilament fineness concentration, in percent, in the coagulating bath, maintain- 50 In demers. ing the delivery ratio, defined as the average flow velocity of 3. The acrylic fiber according to claim 2, wherein the conthe spinning solution through the spinning orifice/the pull-out trast index Of the fiber is 15-50 and the 60-degree mirror-survelocity of the coagulated filament, within the range of 0.5 to f c lus er egree of the fiber is at least 35. 3.0, subjecting the filament to cold-stretching at room tem- 

2. An acrylic fiber prepared by a wet-spinning method characterized by a curved peripheral edge in the triangular cross-section of the fiber which edge is defined by the relative formulas wherein l is the length in microns of one side of the fiber represented as a straight line connecting two apexes of the triangular cross-section of the fiber, a is the minimum curve height in microns of the peripheral edge measured in the direction vertical to the straight line connecting the two apexes of the triangular cross-section, p is the minimum curve pitch length in microns of the peripheral edge measured in the direction along the straiGht line connecting the two apexes of the triangular cross-section and d is the monofilament fineness in deniers.
 3. The acrylic fiber according to claim 2, wherein the contrast index of the fiber is 15- 50 and the 60-degree mirror-surface luster degree of the fiber is at least
 35. 